1) Field of the Invention
The present invention relates to a sheet separating mechanism having a separation claw, a fixing device, and an image forming apparatus.
2) Description of the Related Art
An image forming apparatus includes a fixing device for permanently fixing a toner image on a sheet-like recording medium such as a paper. This fixing device includes a hot fixing roller that is hot, a fixing belt, a pressure roller, and a pressure belt. The fixing roller and the pressure roller are rotatable. The fixing belt is suspended over the fixing roller and the pressure-belt is suspended over the pressure roller. The pressure roller and the pressure belt are in press contact with the fixing roller and the fixing belt; therefore, the pressure roller and the pressure belt rotate when the fixing roller rotates. When the paper with the toner image passes between the pressure roller and the fixing roller and between the pressure belt and the fixing belt; because the fixing roller is hot, the toner melts and gets fixed to the paper.
Sometimes the molten toner, which is a resin, disadvantageously gets adhered to the fixing roller and damages the later images. There are various approaches to avoid the toner getting adhered to the fixing roller. For example, wax may be added to the toner, a mold-releasing material may be coated on the surface of the fixing roller, a mold-releasing material such as silicon oil may be coated on the fixing roller, and so on. Another approach is to provide a sheet separating mechanism having a separation claw that forcibly separates the paper from the fixing roller.
The separation claw has a claw portion and a rotating shaft, and the claw portion is pressured with a spring such that the claw portion is brought into contact with the fixing roller around the rotating shaft. A surface opposing the claw portion with respect to the paper also has a guide shape which directly carries a separated paper.
In the fixing device having such a sheet separating mechanism, when a paper folded like bellows gets jammed near a paper delivery portion, the paper exerts pressure on the separation claw. As a result, the distal end of the claw is pressed on the fixing roller and causes damage to the fixing roller. If there is a layer of an elastic material on the surface of the fixing roller, the distal end of the separation claw breaks into the fixing roller to disadvantageously peel the layer of the elastic material.
Various mechanisms have been proposed for moving the separation claw in such a direction that the separation claw is prevented from breaking into the fixing roller However, there is a problem with those mechanisms in that a lot of parts such as the link mechanisms, springs etc. are required so that the manufacturing cost, or the maintenance cost increases.
In the technology disclosed in Japanese Patent Application laid-open No. 8-272259, a rotating shaft of a separation claw is supported by a groove-like guide to regulate the movement of the separation claw along the groove, and biasing force of a spring for biasing the separation claw to a rotatable member is used to position the separation claw, so that the purpose can be achieved with a simpler configuration.
However, in the sheet separating mechanism described in Japanese Patent Application laid-open No. 8-272259, the separation claw is not positioned until the spring is set in the assembly of the sheet separating mechanism, the separation claw becomes unstable in the setting to deteriorate the assembly properties. Because a tool such as a needle-nose plier is required to set a small spring, reasonable assembling time is required to assemble a plurality of separation claws (in general, 4 to 10 claws). Moreover, when the separation claw moves to twist the rotating shaft, the separation claw may compete with the groove to disadvantageously inhibit the separation claw from smoothly moving.